In a commercial AC transmission and distribution system, a transformer is used. Just near a consumer's house, a pole transformer is used which transforms, for example, 6600 V (50 Hz or 60 Hz) to 200 V (see NON PATENT LITERATURE 1). Such a pole transformer has a thick coil as a conductive wire wound around an iron core, and therefore has a considerable weight. For example, a pole transformer with a diameter of 40 cm and a height of 80 cm has a weight of about 200 kg, including an insulating oil and a case.
On the other hand, for realizing a smart grid which is a next-generation power system, studies of an SST (Solid-State Transformer) are being conducted. For the SST, a high-frequency transformer is used (for example, see NON PATENT LITERATURE 2).